Complex model in Etabs

[Minh102]

Analysing complex model in Etabs (with part of the building in model is not parallels to main axes), I often get warning messages about losses in accuracy. I guess that the reason is Etabs' automesh function were not working properly. How to fix this problem? Anyone have the same problem with me?

 

[WillisV]

Loss of accuracy warnings are normally indicative of improper restraints (too many releases). Make sure that your restraints are not leaving any nodes free to "spin" in the air. Loss of accuracy messages due to automeshing are certainly possible and are normally due to tight corners etc. where ETABS is forced to provide a very atypical (long and slender) mesh to meet the structure. Try "smoothing out" the edge of slab of the building to avoid this problem.

 

[Minh102]

WillisV, you are right. I have noticed that warning massages points to place in model where two beams intersect with very near but not equal 90 degree. I hope next version of ETABS will have more intelligent automesh, but now I have to draw null line to force automeshing which is really time consuming.

Another problem is that ETABS not designs wall resisting forces acting on it surfaces, so I have to design retaining wall at basement level manually. Is there any tip for modelling basement wall?

 

[WillisV]

Minh102,

I typically design basement and retaining walls seperately from the actual analytical model. Normally for basements the forces applied tend to balance themselves out from both sides of the building and there is no need to carry the force in the actual model. In cases where the earth loads are coming from only one face and cannot be effectively taken by the walls on the sides, I normally just placed the load on a ridig diaphragm master node at the center of pressure to distribute it to the lateral system. The actual design of basement walls is easy as they are normally treated as simply supported concrete beams.

 

[Minh102]

Thank you for sharing experience. Like you I often design basement walls as simply supported beams, but sometimes when vertical loads are high I also check wall as columns using P-delta analysis and design under combined load.

 

[WillisV]

I agree Minh102, with high basement loads I will take a look atj more advanced P-delta analysis though this rarely adds more than 10% or so to the controlling moment.

Another alternate is to use the ACI method for slender precast wall elements (new as of ACI-99 - in chapter 11 on walls after the bearing wall empricial method). This method was placed into the code due to the fact that precasters were using taller and more slender wall sections without specific guidance for their design. Though originally inteded for precast it is equally valid for reqular walls and is essentially a p-delta method.